Method of preventing etching by chromic acid chromium plating baths



METHOD OF PREVENTING ETCHING .BY ACID CHROMIUM PLATING T Jesse E. Stareck, Royal Oak, and Ronald Dom-Detroit, Mich., assignors, by mesne assignments, to Metal & Thennit Corporation, New York, N. Y., a corporation of New Jersey No Drawing. Application March 13, 1953, Serial No. 342,292

11 Claims. (Cl. 204-34) This invention relates to the prevention of etching of metal articles that are to be chromium plated.

Etching of metal articles during plating in chromic acid baths is undesirable because it involves a dissolution of the metal with consequent damage to the metal surface, and it results in the contamination of the plating solution. Sometimes etching produces discoloration of an article, which is a disadvantage wherever the appearance of the plated article is important. Etching is particularly a problem in the plating of articles having recessed or unprotected areas where no chromium is deposited.

.It is commonly known that chromicacid chromium plating baths having a high catalyst acid radical content tend to produce etching on unplated areas which are exposed to the plating solution. For example, in conventional CrOs-SOt baths etching may occur at chromic acid/sulphate ratios of 75:1 and lower, with the etching becoming more pronounced at the lower ratios. When other catalyst acid radicals such as chloride, fluoride or fluoride-containing radicals are used, the tendency to etch is increased. A commonly used fluoride-containing radical is the silicofiuoride, as in the well-known CrOa- SOt-SiFs bath.

A principal object of the invention is to minimize or eliminate the tendency of chromic acid plating bathsto etch by providing a method of preparing or pretreating the article to be plated so as to coat it with an etchresistant film on at least those exposed areas that will not receive any chromium plate. The filmed article is then plated, it having been found that the film so protects the article against the effects of the bath as to reduce or eliminate etching on the areas covered by thefilm.

Filming of the article is done by immersing it, or at least that portion of it which is to be protected against etching, in an aqueous solution containing about 100 to 1000 g./l. of chromic acid maintained at a temperature nited rates Patent of about 50 to 110 F. and substantially free of catalyst acid radicals. A current density in the range of 0.05 to 2 a. s. i. is passed from an anode through the solution to the article as a cathode for a period of time ranging from 0.5 to 10 minutes. In these ranges, higher current densities are used with lower times and CIOs concentrations, and vice versa. Preferred filming conditions comprise the use of an aqueous solution of about 300 to 500 g./l., particularly about 400 g./l., of CrOs at room temperatures and a current densityof 0.1 to 1.0, particularly 0.5 a. s. i. for a time of 2 to 5 minutes. No chromium is plated on the article during this treatment and to insure that none is plated and that the required film is formed on the article, it is desirable to have the chromic acid solution free or substantially free of catalyst acid radicals. However, in some cases the use of phosphoric acid in amounts ranging from 75 to 300 g./l. in the chromic acid filming solution is desirable. i

The filming solution may beconveniently prepared, for example, by dissolving in water ordinary chromic acid of technical grade. If it is known that thechromic acid,

from whatever source, contains more than about 0.1% sulfate (the usual amount in the technical. grade chromic acid is about 0.1% by weight), the sulfate is removed by chemical precipitation as barium sulfate; and such removal is also done in regard to the filming solution if the sulfate concentration builds up during use. Any other catalyst acid radicals which may be present as im purities are also removed.

The film that is produced on the article may be almost invisible when wet; it may vary from one that is thin and transparent to one that is thicker and has a colored cast, such colors as brown, olive drab, yellow, green, and grey being found. The colored films are usually iridescent. Besides affording protection against etching, the film improves the covering or throwing power in the chromium plating operation. In effect, it acts to extend the current density range of the plating bath. If it is desired, the film may be removed from the article following the plating step, and this may be done, for example, by means of a quick dip in dilute hydrochloric acid, or by an anodic treatment in alkali or chromic acid solution, or by an abrasive action as .by wiping on a polishing wheel.

Prior to filming, the article is cleaned by the usual methods and subjected to a conventional anodic treatment, the latter, as is apparent, also being a cleaning step. Then the article is filmed and plated. It is preferred, but not necessary, to perform the anodic treatmentand the filming in the same filming solution.

Examples 1, 2, 3, 4 and 5 illustrate how films may be produced, and Examples 2, 3, 4 and 5 show that they are protective; Example 4 further demonstrates the protective nature of the films in terms of weight loss.

Example 1 Three solutions were made up, identified as Nos. 1, 2, and 3, containing, respectively, 100, 300 and 600 g./l. of CrOs. A flat steel strip 1" x 6 was immersed in each of Nos. 1, 2 and 3 at F. and a current density of 0.05 a. s. i. passed from an anode to the strip as cathode for 5 minutes. The strips were removed, washed, dried, and inspected. The strip from solution No. 1 had no film, that from No. 2 had a very thin, transparent film, and that from No. 3 had a yellowish-green iridescent film. The foregoing tests were repeated using 0.1 a. s. i.,

. and it was found that the strip from No. 1 had a light olive drab iridescent film, that from No. 2 wasthe same as the. latter except thatits film was thicker, and that from No. 3 had. a dark olive drab film. A repetition of the tests using 0.50 a. s. i. produced strips having olive drab films in the case of Nos. 1 and 2 and a light grey film in the case of No. 3. In another series of tests using 0.05 .a. s. i. and a temperature of F., no films were produced on any of the strips, indicating that the higher temperature had decreased the tendency for a film to form. A further series of tests at 110 F. but with 0.10 a. s. i. produced on the strip from No. 1 a very thin, olive drab, iridescent film; on the strip from No. 2 an olive drab, iridescent film; and on the strip from No. 3 an olive drab, non-iridescent film.

Example 2 etching action of the plating solution. Nevertheless, the unplated back side of each panel was not etched and was I not on the outside.

.I 3 thus protected by the film. An .unfilmed .control panel, plated under identical conditions, became etched on its unplated back side.

Example 3 Two solutions, identified as Nos. 4 and 5, were made up containing 200 g./l. of CrOa and 100 cc. per liter of phosphoric acid, and 400 .g./l. CrOs and 100 cc. per liter phosphoric acid. Ordinary, commercial grade phosphoric acid was used, 100 cc. of which weighed 143.6 g. A steel tube 4% long x 2%" diameter was immersed in each solution at 75 F. and treated as cathode using a current density of 0.5 a. s. i. for 5 minutes. The surfaces of the tube from No. 4 had a semi-bright metallic appearing film inside and outside, while the tube from No. 5 had a dull r olive drab film. When plated in a chromium plating solution of the chromic-acid-sulfate-silicofiuoride type using only an inside anode for 2 hours at 135 F. and '2 a. s. i.,

both tubes were plated with chromium on the inside but was exposed to the etching action of the plating solution, it'was not'etchecl. An unfilmed control tube, plated under the foregoing conditions, was etched on the outside.

Example 4 Two flat steel strips 1 x 6" were immersed for 5 minutes at 80 F. in a chromic acid solution containing 400 g./l. CrOs and a current density of 0.5 a. s. i.'was

passed to produce a film on the strips. In a second step,

the filmed strips were successively subjected as cathode to-the action of a chromium plating solution of the chromic acid-sultate-silicofluoride type for one hour at 130 F.

at current densities of 0.02 and 0.05 a. s. i., respectively. The loss in weight of the strips is tabulated below. For

comparison, the losses obtained in treating two identical but unfilmed strips only according to the above'second step are also given.

Current density, a.s.i 0. 02 0. 05

Weight loss. grams:

Filmed strip 0. 043 O. 004 Unfilmed strip 0. 323 0. 380

As is apparent, the filmed strips have very substantially reduced losses compared to the control.

Example 5 Two steel panels 3" x 4", one of them filmed using .the

.filming conditions set forth .in Example 4 and the other .unfilmed, were subjected as .cathode to the action of a 'chromicacid-sultate type chromium .platingbath at 140 F. for 1 hour, using 0.1 a. s. i. The unfilmed panel was etched on bothsides while the filmedpauel was .not etched on eitherside.

The invention is suitable for articles made of steel,

particularly of plain carbon steels of low, medium, and high carbon content, and of steels commonly used in tools.

Qt'ion is applicable also to non-ferrous metals like nickel,

zinc, copper, brass, and bronze.

In the light of the foregoing descriptionthe following is claimed: 7

1. Method of preventing etching of a metal article to be chromium plated in a chromic acid plating bath containing acid radicals, said bath tending toetch non-plated,

exposed areas of the article which comprises immersing at least that portion of the article to be protected against etching in-an aqueous solution consisting essentially of Although the outside of each tube a .100 to 1000 g./l. of CrOs for a period of time ranging from 0.5 to 10 minutes and at a temperature ranging from 50 to 110 F., passing current in the range of 0.05 to 2 amperes per square inch .froman anode to said article as a cathode during said time, thereby forming an etchresistant'filmon the article, said filming step beinginoperati-ve to deposit chromium plate on the article, then placing the article in said chromic acid chromium-plating bath andplating chromium thereon, said film protecting non-plated exposed areas of the article againstetching by said chromic acid plating bath.

2. Method according to claim 1 in which the metal article is a steel article.

3. Method of preventing etching of a metal article to be chromium plated in a chromic acid plating bath containing catalyst acid radicals, said bath tending to etch non-plated, exposed areas of the article which comprises immersing at least that portion of the article to be protected against etching in an aqueous solution consisting essentially of 100 to 1000 g./l. of CIO3 and 75 to 300 g./l. 'of phosphoric acid for a period of time ranging from 0.5

"to 10 minutes and-at a temperature ranging from 50 to '110F.,'passingcurrent in the range of 0.05 to 2 amperes per squareinch'from an anode to said article as a cathode during said time, thereby forming an etch-resistant film 'on the article, said filming step being inoperative to deposit chromium plate on the article, then placing the article in said chromic acid chromium plating bath and plating chromium thereon, said film protecting non-plated exposed areas 'of the article against etching by said chromic acid 4. Method of protecting a metal article against etching during chromium plating of the article in a chromic acidfwhich comprises immersing at least that portion of the article to beprotected against etching in an aqueous solution consisting essentially of 300 to 500 g./l. of CrOs for a period of time ranging .from 2 to 5 minutes and at room "temperature, passing current in the range of 0.1 to 0.5 ampere per square inch from an anode to said article as a cathode during .said time, thereby forming an etchresistant film on the article, said filming step being inoperative to deposit chromium plate on the article, then placing the article 'in said chromium plating bath and plating chromium thereon, said film protecting non-plated exposed areas of the article against etching by said plating 'bath.

5. Method according to claim 1 in which the plating bath is of the chromic acid-sulfate type.

6 Method according to claim 1 in which the plating bath. is ofthe chromic acid-sulfate-silicofiuoride type.

7. Method of preventing etching of a metal article to be chromium plated in a chromic acid plating bath containing catalyst acid-radicals, said bath tending to etch nonplated, exposed areas of the article which comprises immersing at least 'thatportion of the article to be protected against etching in an aqueous solution consistingessentially of to 1000 g./l. of CrOa for a period of time ranging from 0.5to'10 minutes and at a temperature ranging from 50 to F., said solution being substantially free of catalyst acid radicals, passing current in the range of 0.05 to 2 amperes per square inch from an anode to said article as a cathode during said time, thereby forming an etch-resistant film on the article, said filming step being inoperative .to deposit chromium plate on the article, then immersing the filmed article in said chromic acid plating bath and depositing chromium plate thereon, said film protecting non-plated, exposed areas of the article against etching "by said chromic acid plating bath.

8. Method of protecting 'ametal article against etching during 'chromium'platin'g of the article in a chromic acid plating bath containing catalyst acid radicals, said bath "tending to etch non-plated, exposed areas of the article which comprises immersing at least that portion of the article to be protected against etching in an aqueous solution consisting essentially of 100 to 1000 g./l. of CrOa for a period of time ranging from 0.5 to 10 minutes and at a temperature ranging from 50 to 110 F., said solution being substantially free of catalyst acid radicals to help avoid deposition of chromium from said solution on said article, passing current in the range of 0.05 to 2 a. s. i. from an anode to said article as a cathode during said time to form an etch-resistant film but not deposit chromium on said portion of the article during the passage of said current, then placing the filmed article in said chromic acid chromium plating bath and depositing chromium plate thereon, said non-plated filmed portion receiving no plate but being exposed to said plating bath, and recovering the chromium plated article having said non-plated portion in unetched condition with said film thereon.

9. Method ot claim 8 in which said article, prior to said film forming step, is cleaned by a conventional anodic cleaning step.

10. Method of claim 9 in which said anodic cleaning step is performed in said aqueous, film forming solution.

11. Method of claim 8 in which, following said chromium plating step, the film on said non-plated portion of the article is removed.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Haring et al.: Electrodeposition of Chromium From 20 Chromic Acid Baths, Bureau of Standards Bulletin No.

346 (1927), pages 346 and 347. 

1. METHOD OF PREVENTING ETCHING OF A METTAL ARTICLE TO BE CHROMIUM PLATED IN A CHROMIC ACID PLATING BATH CONTAINING ACID RADICALS, SAID BATH TENDING TO ETCH NON-PLATED, EXPOSED AREAS OF THE ARTICLE WHICH COMPRISES IMMERSING AT LEAST THE PORTION OF THE ARTICLE TO BE PROTECTED AGAINST ETCHING IN AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF 100 TO 1000 G./L. OF CRO3 FOR A PERIOD OF TIME RANGING FROM 0.5 TO 10 MINUTES AND AT A TEMPERATURE RANGING FROM 50 TO 110*F., PASSING CURRENT IN THE RANGE OF 0.05 TO 2 AMPSERES PER SQUARE INCH FROM AN ANODE TO SAID ARTICLE AS A CATHODE DURING SAID TIME, THEREBY FORMING AN ETCHRESISTANT FILM ON THE ARTICLE, SAID FILMING STEP BEING INOPERATIVE TO DEPOSITE CHROMIUM PLATE ON THE ARTICLE, THEN PLACING THE ARTICLE IN SAID CHROMIC ACID CHROMIUM PLATING BATH AND PLATING CHROMIUM THEREON, SAID FILM PROTECTING NON-PLATED EXPOSED AREAS OF THE ARTICLE AGAINST ETCHING BY SAID CHROMIC ACID PLATING BATH. 